There are two types of articles indefinite "a" and "an" or definite "the". You also need to know when not to use an article. The proper use of article may be sometimes complex, especially when you get into the advanced use of English. You have to work it out by what sounds right, which can be frustrating for a learner. The most important first step in choosing the correct article is to categorize the noun as count or uncount. You cannot say a or an with an uncountable noun. You cannot make an uncount noun plural. You use an uncount noun with no article if you mean that thing in general. You use "the" with an uncount noun when you are talking about a particular example of that thing. Keep it up, dude! Good luck with your studies...

Это на месяц работы, там помимо общих правил полно исключений. http://www.engvid.com/search/?q=a+the&x=0&y=0